3 Answers
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In the good old days /dev/console was System Administrator console. And TTYs were users' serial devices attached to a server.
Now /dev/console and /dev/tty0 represent current display and usually are the same. You can override it for example by adding console=ttyS0 to grub.conf. After that your /dev/tty0 is a monitor and /dev/console is /dev/ttyS0.

@SFun28: if sudo anycommand works, then sudo -i to go root works too. There is no such thing as a Linux/BSD/Unix where you can’t go root. (Then it wouldn’t be Linux/BSD/Unix anymore.)
– Evi1M4chineMar 21 '17 at 14:43

/dev/console is a virtual set of devices which can be set as a parameter at boot time. It might be redirected to a serial device or a virtual console and by default points to /dev/tty0. When multiple console= options are passed to the kernel, the console output will go to more than one device.

/dev/tty0 is the current virtual console

/dev/tty[1-x] is one of the virtual consoles you switch to with control-alt-F1 and so on.

/dev/tty is kind of an alias to the console (physical, virtual or pseudo device, if any) associated to the process that open it. Unlike the other devices, you do not need root privileges to write to it. Note also that processes like the ones launched by cron and similar batch processes have no usable /dev/tty, as they aren't associated with any. These processes have a ? in the TTY column of ps -ef output.

@RonVince I'm afraid I'm not sure what you are asking. Could you rephrase it?
– jlliagreSep 19 '15 at 20:36

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@RonVince Opening /dev/tty (not /dev/tty[1,..,n]) is the Unix (and Linux) standard for a process to write something to the user's terminal. Directly writing to whatever other device is non portable, more complex and less reliable. A process doesn't need to know what is the actual device the process terminal is connected to, the kernel knows it already.
– jlliagreSep 20 '15 at 15:31

/dev/console

On Linux, the kernel console can be configured using the console=boot option. Kernel code which calls printk() may write messages to it, e.g. when a device is loaded or an error occurs. These messages are also buffered by the kernel. (See also dmesg). When a console device is found and started, it receives all the previously buffered messages.

You can pass console= multiple times to configure multiple consoles, and messages will be written to all of them. Apparently you can only select one console of each "type": you can't use both console=ttyS0 and console=ttyS1.

The kernel documentation specifies /dev/console as a character device numbered (5,1). Opening this character device opens the "main" console, which is the last tty in the list of consoles. The first non-kernel process, called init or "PID 1", is started with /dev/console connected to standard output, standard error, and standard input.

If none of the consoles are a tty, then opening /dev/console returns the error ENODEV ("No such device"). The kernel will print log a message, and start init regardless. For an example of a kernel console which is not a tty device, see netconsole, or my favourite console the line printer.

You can also see a list of tty consoles by reading /sys/class/tty/console/active. systemd documentation points out that the first device shown is the main console. The list is actually in reverse order of the kernel command line. The current kernel documentation incorrectly states that the last device shown is the main or "active" console. For some reason it is possible to poll this file for changes (in case console devices are removed?).

Inside a systemd-nspawn container, the standard /dev/console file is replaced with a pseudo-terminal device (PTY). These would be best described as virtual terminal devices. They are created dynamically and are also used to implement graphical terminal emulators like GNOME Terminal, and for remote access like ssh.

/dev/tty0

The Linux TTY device nodestty1 through tty63 are virtual terminals. They are also referred to as VTs, or as virtual consoles. They simulate multiple consoles on top of the physical console device driver. Only one virtual console is shown and controlled at a time. The active terminal can be switched, e.g. using chvt, or Ctrl+Alt+F1 through however many function keys you have.

You can also read and write to the current VT using /dev/tty0. tty0 is the usual kernel console, e.g. if you did not select one explicitly. "The system first looks for a VGA card [which is what VTs run on] and then for a serial port". You can also set the console to a specific VT, e.g. console=tty1.

"If you don't have a VGA card in your system, the first serial port will automatically become the console." A "serial console" like ttyS0 is probably the most common alternative to tty0. It is not possible to use the VT system on top of a serial console.

Detaching from the controlling terminal is one of the steps traditionally required to start a background process, for example a system logging daemon. The steps to become a background process are horribly intricate, but to be specific, the step which detaches from the controlling terminal is the setsid system call. In more modern systems, the init system e.g. systemd starts the service without any controlling terminal in the first place.